1. Technical Field of the Invention
The present invention relates to an engine starter (i.e. starter for engines), and in particular, to a technique for preventing overheating caused in a starter due to continuous current application to a motor possessed by the starter.
2. Related Art
A starter motor described in Japanese Unexamined Published Application No. 11-289724, for example, has a field coil which is connected in series with an armature coil through a brush and a commutator. The field coil is connected to a motor lead through a connector bar. In a starter motor having such a field coil, when a current is continuously supplied due to return failure of a key switch, for example, a problem has been raised that an insulating member in an armature is melted by heat generation in the motor to cause winding of the armature to be brought into contact with the ground or other electric conductor portions. In particular, the problem has been such that the winding, whose insulating member has been melted, unavoidably makes a short circuit between layers of the winding, or makes a short circuit extending to a grounding portion of a yoke or the like, with the result that a large short-circuit current keeps flowing to finally fuse the field coil and also having no control over when it will be fuse or which portion of the coil will be fused. A countermeasure for resolving this problem, i.e. a countermeasure for such abnormal circumstances as when excessively large thermal load is imposed on a starter motor, is to control so as to electrically disconnect a battery from the starter motor on an earlier occasion. Preferably, this fusing issue could occur at a predetermined fusing portion.
In the starter of the type mentioned above, however, resistance of both of a field coil and a connector bar is set at a small level. Therefore, heat generation is not drastic when a large current flows through them at the time of short-circuiting, and thus a circuit is fused only when a certain period of time has passed. In a state where a short-circuit current flows through a field coil, it is important to reduce the time of short-circuiting as much as possible for the sake of safety. To this end, one approach, for example, may be to increase resistance of the entire winding of a field coil for acceleration of heat generation to thereby fuse the winding of the field coil on an earlier occasion. This approach, however, causes other problems that the fused winding forms a short circuit with adjacent winding without providing complete disconnection, or that a certain period of time is required to achieve complete disconnection, or that output is reduced in the motor.
To resolve such problems, Japanese Unexamined Published Application No. 2005-110484 describes a stator motor, in which electrical connection is established between a motor lead for receiving current supply for engine start from a battery through a battery cable, and a metal intermediate member disposed in an internal circuit of the motor, so that, upon imposition of an excessively larger thermal load on the motor circuit than ordinarily used, a fuse function, i.e. a function of fusing the intermediate member is exerted to shut down the motor circuit. In order to improve the function as a fuse, the material of the intermediate member has selectively been iron, aluminum or tin, for so example, which has higher electrical resistance and lower thermal conductivity than wiring (typically a copper wire) connected to the intermediate member.
However, providing such an intermediate member, i.e. a member a starter originally did not have, raises some problems. For example, the number of parts is increased. Also, the number of steps is increased, such as, a step of welding the intermediate member to the wiring, or a step of applying a surface treatment (e.g., tin plating) to the intermediate member to improve weldability. Furthermore, design of some members other than the intermediate member have to be changed in order to ensure a space for accommodating the intermediate member.